As the earth's temperatures rise, some have speculated that trees and shrubs in the colder climates might experience and increase in growth as a result of the extended growing season. "Not so," says a recent study authored by a University of Washington biology and applied mathematics postdoctoral student. Her study demonstrates that bushes achieve less yearly growth when cold winter temperatures are interrupted by warm spurts that trigger growth.

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"When winter temperatures fluctuate between being cold and warm enough
for growth, plants deplete their resources trying to photosynthesize and end
the winter with fewer reserves than they initially had. In the summer they have
to play catch up," said lead author Melanie Harsch. The paper was recently
published in PLOS ONE.

Harsch says the roots are especially sensitive to temperature
fluctuations and warming winters result in higher root respiration. Higher root
respiration uses up the plant’s carbon making less available during the regular
growing season.

The study of two species of shrub was conducted on Campbell Island, an
uninhabited UNESCO World Heritage site in the southwest Pacific Ocean 375 miles
south of New Zealand. The two shrubs, Dracophyllum longifolium and Dracophyllum
scoparium, are evergreen broadleaf plants that reach about 15 feet and can
live up to 240 years.

"For growth to occur you need sufficient precipitation and
temperature and nutrients. Growth should only happen during the summer on
Campbell Island when temperatures are above 5° Celsius (40° Fahrenheit) Harsch
said. "On Campbell Island most winters are cool and below this 5° Celsius,
so the plants are not active. The plants we studied are evergreen and there is
little snow cover, so they are sensitive to changes in temperature."

To
conduct the study researchers cut out discs, called "cookies," from
just above the shrubs' root collar, measuring the widths between each ring to
determine growth. Plant growth decreased with the rise in winter temperatures.

"On Campbell Island the snow is ephemeral, so the plants usually
are not covered," Harsch said. "If we're going to see an effect in
changing winter conditions, we're going to see it at Campbell Island decades
before we see it at, say, Mt. Rainier, where there is a lot of snow and winters
are colder."

Harsch said plants in areas like Campbell Island may eventually adjust
to warmer winters, but the transition period will be tough as temperatures
bounce above and below what plants need to stay dormant, causing the plants to
draw down their resources.